A defining feature of a circadian oscillator is that periodic output from the oscillator continues after the termination of periodic input. In contrast, a defining feature of a pacemaker-accumulator system is that elapsed time is measured with respect to the presentation of a stimulus, according to the classic description of this system. Consequently, the output of a short-interval system is periodic if presented with periodic input, but periodic output can be expected to cease if periodic input is discontinued. Groups of rats were trained to time short intervals (1-3 min); periodic delivery of food produced periodic behavior. Next, delivery of food was suspended. Behavior was periodic after termination of periodic input, and the period in extinction increased as a function of the period in training. These data suggest that short-interval timing is, at least in part, based on a self-sustaining, endogenous oscillator.